Nitrous oxide and methane during the 1994 SW monsoon in the Arabian Sea/northwestern Indian Ocean

Partial pressures of dissolved and atmospheric nitrous oxide, N2O, and meth ane, CH4, were measured during the latter stages of the southwest (SW) mons oon and subsequent intermonsoon transition of 1994 in the Gulf of Oman and the northern and central Arabian Sea, during Discovery cruises D210 and D21 2 of the United Kingdom Joint Global Ocean Flux Study of the northwestern I ndian Ocean (NWIO). Mean observed atmospheric mixing ratios, 310 +/- 3 ppbv N2O, 1706 +/- 17 ppbv CH4 (SW monsoon), and 311 +/- 3 ppbv N2O, 1784 +/- 2 0 ppbv CH4 (intermonsoon), were analytically indistinguishable from contemp orary global baseline data. Mean surface mixed layer saturations were spati ally and temporally heterogeneous. Largest variation was observed for N2O i n an upwelling region adjacent to the Oman coast; mean N2O saturations were 140 +/- 40% (SW monsoon) and 119 +/- 17% (intermonsoon), with correspondin g CH4 saturations of 170 +/- 55% and 179 +/- 15%. These apparent difference s were largely a consequence of less detailed station coverage during D212, reflecting large variability on a relatively small spatial-scale rather th an true seasonal variation; for individual stations in the coastal upwellin g, temporal changes in mean mixed layer saturations were not significant. T his suggests that within this region the processes of gas exchange, net pro duction, and supply/removal by advection and vertical mixing were more or l ess in balance during the period studied. Open ocean saturations were lower and less variable: 106 +/- 7% N2O, 130 +/- 5% CH4 (SW monsoon) and 104 +/- 6% N2O, 115 +/- 2% CH4 (intermonsoon). Large supersaturation maxima for N2 O (saturations similar to 400-800%) and CH4 (saturations similar to 200-400 %) just below the base of the mixed layer were ubiquitous and followed a tr end of progressive deepening toward the south. All deep N2O profiles were c haracterized by a second, more vertically extensive maximum (saturations si milar to 400-600%) between 500 and 1000 m. For these, plots of Delta N2O ve rsus AOU were consistent with their formation by coupled nitrification-deni trification, with denitrification becoming progressively more important wit h distance toward the core of the oxygen depleted zone. Sea-to-air fluxes f or a 6-month period represented by the study were determined from measured air-sea partial pressure differences and gas transfer velocities derived fr om in situ wind speeds. Estimated semiannual emissions, similar to 0.41-0.7 5 x 10(12) g N2O, similar to 0.1-0.18 x 10(12) g CH4, were within most prev iously reported ranges for the NWIO. The data indicate that seasonal change s in wind speed rather than seasonal changes in air-sea partial pressure di fferences due to monsoon-driven mixing and upwelling are the dominant contr ol on air-sea gas exchange in the NWIO.